Finding my computer at home from the outside.

1. Introduction
2. Perl script uploading ip address.
3. Webpage and perl script on remote computer.
4. Cron job I run in the background.
5. Conclusion
6. References

Introduction

Some people can have static DSL connections, which takes of the problem I have, which is my ip address to the internet changes each time I dial up. I used to email myself the ip address, parse out the data, and put it on a webpage. I have a better solution now. I use ssh to transfer a file to my remote webserver once an hour.

Setting up ssh.

We need to make it so we can transfer files securely from my computer at home to the remote computer. We use the ssh-keygen program (which comes with ssh). Here is a paragraph from the manpage for ssh.

       Ssh  implements  the RSA authentication protocol automati­
       cally.  The user creates his/her RSA key pair  by  running
       ssh-keygen(1).   This stores the private key in .ssh/iden­
       tity and the public key in .ssh/identity.pub in the user's
       home  directory.   The  user  should  then  copy the iden­
       tity.pub to .ssh/authorized_keys in his/her home directory
       on  the  remote  machine  (the authorized_keys file corre­
       sponds to the conventional .rhosts file, and has  one  key
       per line, though the lines can be very long).  After this,
       the user can log in  without  giving  the  password.   RSA
       authentication is much more secure than rhosts authentica­
       tion.

rsync -e ssh -av /home/test1/IP.txt web1@somecomputer.com:public_html/IP.txt

Perl script uploading ip address.

#!/usr/bin/perl

use strict;

  ### Run ifconfig and store the data in the @Temp list. 
my @Temp = `/sbin/ifconfig`;

  #### Search for ppp
my $Search = "ppp";
  ### If you are looking for the ip address of your ethernet card, 
  ### uncomment the next line;
# $Search = "eth0";

  ### Make the line we find the ip address blank initially.
my $Match_Line = "";
my $Match_Device = "no";

  ## Search through the lines, if we find a match, save the lines until
  ## we find a blank line. 

foreach my $Line (@Temp)
  {
    ### If we have a match, abort. 
  if ($Match_Line ne "")   {@Temp = ();}
    ### else, see if we can find a match at the beginning of line;
  elsif ($Line =~ /^$Search/) {$Match_Device = "yes";}
    ### else, if we found the device, and we find the line we are looking for
  elsif (($Match_Device eq "yes") && ($Line =~ /^ +inet/)) 
    {$Match_Line = $Line;}  
  }

  ## If our $Match_Line is not blank, split it and get the ip address.
my $IP = "";
if ($Match_Line ne "") 
   {
    ### Get rid of stuff before addr:
   my ($Junk,$Good) = split(/addr\:/, $Match_Line,2);
    ### Get rid of stuff after the first space
   my ($Good,$Junk) = split(/ /, $Good,2);
   $IP = $Good;
   }

  ## If $IP is not blank, we have something. Save to file and transfer file
  ## to remote site. 
  ### Please don't use the /tmp to store this file, but some other location.
if ($IP ne "")
  {  
  open(FILE,">/tmp/IP.txt");
  print FILE "$IP\n";
  close FILE;
  system ('rsync -av -e ssh /tmp/IP.txt web1@somecomputer.com:public_html/IP.txt');
  }
   ### Else, we should send ourselves an email, or do something
   ### to let us know it didn't work. This is left as an exercise.
else {}

Webpage and perl script on remote computer.

You may want to consider moving this perl script into the normal cgi-bin directory of your webserver. Otherwise, here is a dangerous example of how to make it so you can run perl scripts from a users directory. THIS IS DANGEROUS! If your web server allows any user to execute a perl script, that person can get the webserver to do anything they want.

To make it so you can execute perl scripts on your webserver,

<Directory /home/*/public_html>
   ## Options All is reduntant with some of the other options. 
    Options All Indexes FollowSymLinks MultiViews ExecCGI Includes 
    AllowOverride All
    Order allow,deny
    Allow from all
</Directory>

   #### This requires several perl apache modules
 <Files *.pl>
 SetHandler perl-script
 PerlHandler Apache::OutputChain Apache::SSIChain Apache::Registry 
 PerlSendHeader On
 Options ExecCGI
 </Files>

#!/usr/bin/perl

use strict;

print "Content-type: text/html\n\n\n\n";

my $File = "/home/web1/public_html/IP.txt";
open(FILE,"/home/web1/public_html/IP.txt");
my $Line = <FILE>;
chomp $Line;
close FILE;

my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
   $atime,$mtime,$ctime,$blksize,$blocks)
     = stat($File);
my $time = time();

print "<br> Last known ip address was $Line\n";
print qq(<br> To transfer to the website, 
     <a href="http://$Line">click here</a>\n);

my $Diff = $time - $mtime;
if ($Diff > 4000) 
  {
  print "<p>ERROR: The ip address should have been updated once an hour, 
  but 4000 seconds has past since the last update.
  <br> $time - $mtime = $Diff \n";
  }

The Cron Script to make it run nightly

#/bin/sh

### Crontab file
### Name the file "Crontab" and execute with "crontab Crontab"
### Or, you can edit the current crontab with 'crontab -e' and then put
### the lines below in it -- which is safer. 

  ### Download every two hours
1 * * * *   /www/Cron/Remote_Website.pl >> /www/Cron/out  2>&1  

Conclusion

Thanks for Mike Orr for suggestions!

References

1. ssh
2. OpenSSH
3. Apache
4. If this article changes, it will be available here http://www.gnujobs.com/Articles/17/Remote_Website.html

Mark works as an independent consultant donating time to causes like GNUJobs.com, writing articles, writing free software, and working as a volunteer at eastmont.net.

Copyright © 3/2001 Mark Nielsen
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